Optical fiber polarization controller

ABSTRACT

An optical fiber polarization controller which has compact size by employing wave plates made of short sections of a first optical fiber having inherent birefringence such as a polarization maintaining optical fiber. In the control of the polarization state of input light, a second optical fiber, for example a single mode fiber, connected to the first optical fiber is twisted using fixing knobs, thereby rotating the birefringence axis of the first optical fiber with respect to the second optical fiber.

TECHNICAL FIELD

The present invention relates to an optical fiber polarizationmanipulating apparatus, more particularly to an optical fiberpolarization controller which is very compact in size by using shortpieces of birefringent fibers as waveplates.

BACKGROUND ART

Light wave is an electromagnetic wave consisting of the electric andmagnetic fields. As it propagates, the electric and magnetic fieldsoscillate on a transverse plane normal to the propagation direction witha specific oscillation pattern. In general, the polarization is a fieldwhich is parallel to the electric field vector E. Therefore, the stateof polarization is referred to the oscillation pattern of the electricfield on the transverse plane. The state of polarization falls intothree categories in terms of the direction and phase of twomutually-orthogonal components of the electric field vectors E. Thecategories are linear, circular, and elliptical polarization,respectively.

The state of polarization in changed when light propagates through abirefringence medium which has different refractive-indices between twoorthogonal eigen axes. The amount of birefringence, or the indexdifference, is the characteristics of the material itself, however it isaffected by external perturbations such as stress, strain andtemperature. Silica material is inherently birefringence-free because ofits amorphous nature. However, optical fibers that are made of silicatend to exhibit non-negligible birefringence owing to the internalstress as well as non circular-symmetric geometry. Additionalbirefringence can be also induced by external lateral stress or bending.The change of such birefringence by external perturbations which isoften non-deterministic can cause severe problems in fiber-opticapplications such as optical communications and sensors. The effectsinclude polarization-induced signal fading or degradation due to thechange of polarization state. It is therefore very important infiber-optic applications to maintain or control the polarization stateof light. A polarization controller is an apparatus used to convert aninput polarization state to an arbitrary output polarization state and,therefore, a key element in lab experiments, fiber-optic sensors,optical communications, and especially for a system which uses highlypolarization dependent optical devices. For example, high-speed opticalsystem employs lithium niobate(LiNbO₃) as an external modulator toreduce wavelength chirping that comes in at directly modulated lightsource. In this case, due to the high-polarization dependence of themodulator, matching the polarization state to the birefringence axis ofthe modulator is essential to get the best performance. In matching thepolarization state between a laser diode(“LD”) and the externalmodulator, polarization maintaining(“PM”) fiber is generally used toconnect the LD to the external modulator. However, this requires complexprocess of aligning polarization axis of fiber to those of the LD andthe external modulator.

The principle of the polarization controller is that desiredpolarization state is obtained by using appropriate phase retarderswhich can transform a state of polarization(“SOP”) to another SOP. Twoquarter-wave plates can be used for the phase retarders.

FIG. 1 illustrates the change of SOP on Poincare Sphere which isgenerally accepted way to describe the SOP. The convention is that alinear polarization state can be represented by a point “c” located onthe equator, and circular polarization by a point “d” located on thepole of the Poincare sphere. A point on the sphere corresponds to a SOP.Since two orthogonal axes of a coordinates can describe any point on thesphere, a point on the sphere can be moved to another point by rotatingthe two orthogonal axes of the coordinate. This means that any SOP oflight can be transformed to any other SOP by means of rotating the axes.It is well-known that two quarter-wave plates that are used in apolarization controller, for example bent fiber loops for making phaseretardation by inducing birefringence therein, can perform the abovedescribed two rotating axes.

When two quarter-wave plates are used for a polarization controller, therespective azimuthal and polar angles of a point on the sphere can berotated by rotating two axes of the sphere, namely, the optical axes oftwo quarter-wave plates. Therefore, input SOP “a” can be transformed toa desired output SOP “b” as shown in FIG. 1.

FIG. 2 schematically shows a conventional optical fiber polarizationcontroller according to a prior art.

The polarization controller shown in FIG. 2 is disclosed by H. C.LeFevre, in Electronics Letters, Vol. 16, No. 20, September 1980.Referring to FIG. 2, a length of single mode optical fiber 1 is wound ona bobbin 10 with a predetermined diameter. The bending givesbirefringence to the optical fiber by bending-induced stress, whichmakes two birefringence axes, parallel and perpendicular to bobbin 10.At an appropriate diameter, the induced birefringence makes aquarter-wave plate for a given optical wavelength. Since thebirefringence principal axis rotates as the rotation of bobbin 10 toR-direction, the SOP of input light P_(in) can be controlled to adesired SOP polarization state in output light P_(out). With thisbobbin, the polarization controller can not avoid comparably largevolume, making it hard to be mounted on a common circuit board.

FIG. 3 is a cross sectional view showing the application of otherpolarization controller of prior arts. This kind of polarizationcontroller can give small size compared with the above described priorart. Referring to FIG. 3, the polarization controller has a screw 32which can contact the outer surface of optical fiber 31. In thepolarization controller of FIG. 3, the polarization state is controlledby the birefringence, induced from the mechanical stress which isapplied to the optical fiber by screw 32. In principle, there should bemeans for acting as two orthogonal rotating axes to control thepolarization state as described in FIG. 1. The means to achieve thisaction is the screw that presses the optical fiber from differentdirections with different force, which corresponds to the two orthogonalaxes of the above polarization controller. For example, as shown in FIG.3, after the stress applied in X-Y direction is released, other stressis applied in X′-Y′ direction. This single controlling means may causedifficulty in controlling the polarization. The problem with suchpolarization controller is that the reliability of the polarizationcontroller is significantly affected by the squeezing of the fiber,because the squeezing can damage the jacket of the fiber and fiberitself. Moreover, the mechanically squeezed jacket may not recover itsoriginal form so that the stress still remains in the fiber, whichresults in uncontrollable situation.

DISCLOSURE OF INVENTION

It is therefore an object of the present invention to provide an opticalfiber polarization controller with meaningfully small size applicable toan electric circuit board.

It is other object of the present invention to provide an optical fiberpolarization controller having improved durability without squeezing theoptical fiber.

It is another object of the present invention to provide an inexpensiveoptical fiber polarization controller without using PM optical fiber forentire fiber strand of the controller.

In order to accomplish the aforementioned object, the present inventionprovides an optical fiber polarization controller, comprising: at leastone part of a first optical fiber having birefringence, a part of asecond single-mode optical fiber having at least one connected pointwith the first optical fiber to the end of the part to form a strand ofoptical fibers for transmitting light along the strand; and a twistingmeans for controlling angle of the birefringence axes of the firstfiber.

The connected points can be formed by fusion splicing and/or physicalcontact. The optical fiber polarization controller may be configured tohave more than two parts of the first optical fiber.

Each of the parts may have a length adjusted to perform a quarter waveplate according to the difference between its birefringence indices.

According to other aspect of the invention, the optical fiberpolarization controller comprises: two slices of a first optical fiberhaving birefringence; two parts of a second single-mode optical fiber,each part having a connected portion to the end of the slice to transmitlight with the slices; a pair of ferrules for inserting the connectedportions and the slices therein, and for aligning two parts of the firstsingle-mode optical fiber by contacting the facing ends of two slices; asleeve for inserting the pair of ferrules to fix it therein; and arotating means for ferrules.

The connected portions may also be formed by fusion splicing or physicalcontact. Each of the slices can have a length adjusted to perform aquarter-wave plate according to the difference between its birefringenceindices.

The structure of the connected portions is not limited to theaforementioned structure, and any structure can be used if lowconnection losses are guaranteed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the change of polarization state on Poincare Spherewhich is frequently used to show the polarization state of light;

FIG. 2 schematically shows a conventional optical fiber polarizationcontroller according to a prior art;

FIG. 3 is a cross sectional view showing the application of otherpolarization controller of prior arts;

FIG. 4 is a partially enlarged view showing only optical fibers in theoptical fiber polarization controller according to an embodiment of theinvention;

FIG. 5 shows other embodiment of the invention with a more compact sizethan the one in FIG. 4;

FIG. 6(a) to FIG. 6(c) schematically show a fixing apparatus to mountthe optical fiber part of the optical fiber polarization controller on aelectric circuit board.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed referring to the accompanying drawings.

FIG. 4 is a partially enlarged view showing only optical fibers in thein-line optical fiber polarization controller according to an embodimentof the invention.

Referring to FIG. 4, the end portions “f” of slices 110 and 120 areoptically connected to the second optical fiber 100 to form a strand ofoptical fibers capable of transmitting light. The slices 110 and 120 ofthe first optical fiber are birefringent, that is, have two birefringentaxes of different refractive indices from each other. A conventionalsingle mode fiber with low birefringence can be used as the secondoptical fiber 100. The connection between the second optical fibers 100and slices 110 and 120 can be made by fusion splicing. Other connectingmethod includes mechanical splicing or physical contact aided by aferrule. Slices 110 and 120 are short sections of a birefringent opticalfiber with proper length to perform the function of wave plates.Typically, the length can be adjusted to act as a quarter-wave plate orhalf-wave plate. For example, when the index difference(Δn) between thebirefringence axes of the slices is an order of 10⁻⁴ as in thisembodiment, the length of the slices 110 and 120 would be only a fewmm's for an optical wavelength of about 1.5 micrometer. When an opticalwave with a certain polarization state transmits through single ormultiple birefringent slices, the output polarization state isdetermined by the settings of the birefringence axes of the slices withrespect to each other and also to the input polarization state.Therefore, by rotating the birefringence axes, any polarization state inthe input light can be transformed to any polarization state in theoutput. To do this, one can rotate the slice with respect to thebirefringent slices spliced to them. If the slices are mechanicallyconnected to the lead fiber by using a ferrule, the slices can berotated with respect to the lead fibers to change the orientation of thebirefringence axes. Other means includes lateral stress applied to theslices to change the magnitude of the birefringence.

FIG. 5 shows other embodiment of the invention with a more compact sizethan the one in FIG. 4. The piece of the second optical fiber betweenthe optical fiber slices 110 and 120 shown in FIG. 4 is not necessary.In this embodiment, two slices of the birefringent first optical fiber,502 and 504, are connected to pieces 500 of the second single modeoptical fiber by fusion splicing. Then, the yet free ends of the slices502 and 504 are connected to each other by a mechanical splicing orphysical contact. For ease of this splicing process, conventionalphysical contact based on ferrules and sleeves can be used.Specifically, the slices 502 and 504 are inserted into ferrules 506 and508, respectively. The ferrules 506 and 508 are aligned to each otherfor minimum optical loss with help of a cylindrical sleeve 510. Thematerial for ferrules 506 and 508 was zirconia in this particularembodiment, however could be stainless steel, quartz, alumina, or thelike. The first optical fiber slices 502 and 504 are physicallycontacted without using fusion splicing. The typical optical loss of thephysical contact could be less than 0.2 dB.

FIG. 6(a), 6(b) and 6(c) schematically show a fixing apparatus fromdifferent angles which is used to mount the optical fiber elements ofthe optical fiber polarization controller. FIG. 6(a) is a side view,FIG. 6(b) is a plane view and FIG. 6(c) is a front view of the assembledapparatus. A strand of optical fiber composed of the first optical fiberslices 110, 120 and the second optical fiber 100 of FIG. 4 passeslengthwise through a hollow cylindrical fixing apparatus 40. Knobs 42are used to fix the position of the cylindrical fixing apparatus 40.Also, the knobs help the process of rotating and fixing at a position ofthe cylindrical fixing apparatus. The above described fixing apparatuscan be miniaturized to a size small enough to be mounted on a electriccircuit board.

The performance of the optical fiber polarization controllermanufactured as above was analyzed. Total insertion loss of thepolarization controller was less than 0.5 dB, and the back reflectionwas far below −60 dB. Arbitrary input polarization state could betransformed to any output polarization state with an excellentpolarization extinction of greater than −45 dB. Moreover, since thesecond optical fiber was a conventional communication grade single-modeoptical fiber, the polarization controller is perfectly compatible withother fiber-optic components and instruments through fusion splicing orconventional connectors. Unlike the prior arts that was comprised of anapparatus for bending optical fibers to induce birefringence, thepolarization controller according to the invention is much more compactin size. Further, the polarization controller according to the inventionis more durable than another prior art in which an optical fiber has tobe squeezed laterally with frequent changes of the magnitude anddirection of the squeezing. Moreover, the polarization controlleraccording to the invention provides inexpensive optical communicationsystems since entire strand of the optical fibers is composed of commoncommunication grade optical fibers instead of polarization maintainingoptical fibers.

What is claimed is:
 1. An apparatus comprising: a sections of a firstoptical fiber with inherent birefringence, the axes of the inherentbirefringence having different refractive indices; a second opticalfiber connected to said section of the first optical fiber to transmitlight from the second optical fiber to the first optical fiber, or fromthe first optical fiber to the second optical fiber; and twisting meansfor rotating the birefringence axes of said section of the first opticalfiber with respect to said second optical fiber, wherein twisting meansapplies only twist to said second optical fiber without giving anysqueezing on said second optical fiber; wherein said section of thefirst optical fiber is sized to provide phase delay equal to single or amultiple of quarter pi(π) radian between two eigen polarization statesdefined by the birefringence axes.
 2. The apparatus of claim 1, whereinthe first optical fiber comprises a polarization maintaining opticalfiber having a polarizing maintaining property resulted from its shape.3. The apparatus of claim 1, wherein the connection between said sectionof the first optical fiber and the second optical fiber is made byfusion splicing or physical contact.
 4. An apparatus comprising: aplurality of sections of a first optical fiber with inherentbirefringence, the axes of the inherent birefringence having differentrefractive indices; a plurality of sections of a second optical fiberconnected to said sections of the first optical fiber to transmit lightfrom the second optical fiber to the first optical fiber, or from thefirst optical fiber to the second optical fiber; and twisting means forrotating the birefringence axes of said sections of the first opticalfiber with respect to adjacent sections of the second optical fiber,wherein twisting means applies only twist to said sections of secondoptical fiber without giving any squeezing on said sections of secondoptical fiber; wherein said sections of the first optical fiber aresized to provide phase delay equal to single or a multiple of quarterpi(π) radian between two eigen polarization states defined by thebirefringence axes.
 5. The apparatus of claim 4, wherein the firstoptical fiber comprises a polarization maintaining optical fiber havinga polarization maintaining property resulted from its shape.
 6. Theapparatus of claim 4, wherein the connection between said sections ofthe first optical fiber and the second optical fiber is made by fusionsplicing or physical contact.
 7. An apparatus comprising: a plurality ofsections of a first optical fiber with inherent birefringence, the axesof the inherent birefringence having different refractive indices; aplurality of sections of a second optical fiber; wherein two or more ofsaid sections of the first optical fiber are connected to each other andthe remainder 3 of said sections of the first optical fiber areconnected to said sections of the second optical fiber; and twistingmeans for rotating the birefringence axes of said sections of the firstoptical fiber with respect to adjacent sections of the second opticalfiber, wherein twisting means applies only twist to said sections ofsecond optical fiber without giving any squeezing on said sections ofsecond optical fiber; wherein said sections of the first optical fiberare sized to provide phase delay equal to single or a multiple ofquarter pi(π) radian between two eigen polarization states defined bythe birefringence axes.
 8. The apparatus of claim 7, wherein the firstoptical fiber comprises a polarization maintaining optical fiber havinga polarization maintaining property resulted from its shape.
 9. Theapparatus of claim 7, wherein the connection between said sections ofthe first optical fiber and the second optical fiber is made by fusionsplicing or physical contact.
 10. The apparatus of claim 7, wherein theconnection between said sections of the first optical fiber is made byphysical contact.
 11. The apparatus of claim 10, wherein the physicalcontact is aided by ferrules and sleeves.